Allow optional dependencies between components

[kozlovsky]

This PR adds an important possibility for components to have optional dependencies.

Basically, it means that a component B can declare: "I don't need to have component A enabled, but if it is enabled, it should be activated before me".

# inside component B:
async def run(self):
    component_a = await self.use(ComponentA, required=False)
    if component_a.enabled:
         foo = component_a.foo
         bar = component_a.bar
     else:
         foo = bar = None

Optional component dependencies are necessary to fix various scripts like run_tunnel_helper.py, as not all components are used in such scripts.

[qstokkink]

Seems fine to merge as-is.

Some suggestions for improvement (not critical/nitpicks):

• Refactor out duplicate code in tests into common functions to reduce duplication.
• Get rid of the loop fixture in these tests. If the fixture is not used (as reported Pylint) it has some hidden side effects that are needed to configure this test (or alternatively: it does nothing). This implies it should've been set somewhere higher up in the fixture hierarchy.

[kozlovsky]

@qstokkink Thanks! I'll refactor the tests code a bit later to remove duplication.
Regarding the loop fixture, you are right, and it is not necessary in these tests. I removed it.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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15.3% Duplication